In a mobile radio system, radio-frequency, digitally modulated signals are transmitted between mobile stations and base stations. When data symbols are being transmitted in this manner via a mobile radio channel, a number of error sources may occur which, overall, determine the bit error rate, which is related to a specific transmission power. For example, one error source may comprise a small frequency error or a small frequency offset during the transmission between the transmission frequency and the reception frequency, which contributes significantly to the bit error rate and must be compensated for. This frequency offset or this frequency error means a frequency error that is caused by there being different oscillator frequencies at the transmission end and at the reception end. This discrepancy in the transmission-end and reception-end oscillator frequencies may be caused, inter alia, by manufacturing tolerances in the oscillators and oscillator crystals that are used. In particular, the oscillators which are used in mobile stations have oscillators which in some cases have relatively major frequency fluctuations, as a result of them being manufactured at as low a cost as possible.
Mobile stations normally have an apparatus that regulates the reception-end oscillator frequency such that the frequency offset that occurs on reception is as small as possible. This control, which is also referred to as AFC (automatic frequency control) cannot, however, follow minor changes in the transmission frequency without any delay, such as those which occur when a base station with units which produce a carrier frequency that changes from data burst to data burst is transmitting to a mobile station. This can lead to a rapidly changing frequency error or frequency offset, with different mathematical signs.
Furthermore, the frequency error may be influenced by the Doppler effect, since any relative movement between a mobile station and a base station leads to a frequency shift in the transmitted signals. It is therefore important to compensate for a frequency error such as this since, without any such frequency compensation, the receiver cannot demodulate the transmitted data with a sufficiently low error probability.
In some cases, a relatively low signal-to-noise ratio of less than 10 dB is used in the GSM mobile radio standard (Global System for Mobile Communications). With this mobile radio station, the noise is thus a significant error source. The GMSK modulation that is used in the GSM standard uses a signal area with the signal points +1 and −1. Since these two signal points have a phase difference of 180°, small phase errors and frequency errors lead to scarcely any increase in the bit error rate. The GSM mobile radio standard is thus relatively robust to frequency errors when using GMSK modulation.
EDGE (Enhanced Data Rates for GSM Evolution) as well as the associated EGPRS packet service (Enhanced General Packet Radio Service) have been defined as an extension to GSM. EDGE uses 8-PSK modulation. 8-PSK modulation uses a signal area with 8 signal points, with the phase difference between the individual signal points being 45°. For this reason, even small phase errors and frequency errors are disturbingly noticeable and cause a significant increase in the bit error rate.
A control loop (“Automatic Frequency Control”, AFC) is used as one standard method to allow frequency correction or frequency compensation to be carried out. This control loop is used to estimate a frequency offset over a lengthy time period. This estimated frequency offset is filtered and compensated for by means of a control filter. In order to allow such estimation of a frequency offset to be carried out, the GSM radio standard provides a logical channel that is provided specifically for this purpose (“Frequency Correction Channel”). Any frequency offset can also be estimated after equalization, with the aid of the demodulated data bits. However, it should be noted that various radio standards also demand that a mobile radio receiver be tolerant to short-term frequency changes.
In the GSM mobile radio standard, the data is transmitted in data bursts, and the frequency error in one data burst may be different, or may vary differently, from the frequency error in a further data burst. The AFC control loop cannot regulate out a frequency offset such as this. In a situation such as this, it is therefore necessary to use a method that estimates and compensates for a frequency offset in a data burst.
Data that is known a priori is used for this purpose in one procedure for estimation of the frequency offset. Data that is known a priori is used as training symbols in a training sequence. A method such as this for automatic compensation for a frequency offset is known from German Laid-Open Specification DE 100 43 743 A1. Automatic frequency correction for the sample values of the received signal is carried out, in the known method, after channel estimation and before channel equalization. The frequency offset in the received signal was determined by evaluation of a cohesive sequence of sample values, which correspond to the training symbols that are transmitted centrally in the data burst. In this case “correspond” means that the only symbols that are transmitted in the data burst which influence the sample values are the training symbols.
In a further procedure, it is first of all possible to estimate all the data bits, and to use these estimated data bits, or to use them and a training sequence, to determine and compensate for a possible frequency offset. All of the data bits are then estimated once again. In a further alternative procedure, it is possible to estimate only some of the received data bits first of all. This section of the estimated data bits, or the estimated bits and a training sequence, is or are then used to estimate a frequency error, and to subsequently compensate for it. Such method procedures for determination of and compensation for a frequency error in a mobile radio system, caused by a Doppler shift, are known from WO 00/54431.
In the case of the known methods for compensation for a frequency error, either the relatively small number of symbols in a cohesive training sequence, or else data symbols that have been detected and demodulated in advance, are used as the raw data for this estimate. Frequency estimation is based on the least square error method and involves a considerable degree of arithmetic computation complexity. A further significant disadvantage of these methods is the complexity for the prior detection that is required and prior demodulation of the data symbols that are used.
The major factor relating to a frequency error is that it causes an increasing phase error in the time profile. The quality of a frequency estimate thus depends less on the amount of data than, in fact, on the position of the data within a data burst. The phase angles of the sample values which are used for estimation of a frequency error thus differ to a greater extent the further apart from one another these sample values under consideration are within the data burst. A training sequence in the GSM radio standard is formed from 26 immediately successive symbols, with these 26 symbols furthermore being located and arranged directly in a row in the centre of the data burst, so that a training sequence such as this is not suitable for precise estimation of a frequency error. The data bits which are adjacent to the training sequence on both sides of it could likewise be used for estimation of a frequency offset, but would first of all have to be detected and demodulated in order to make it possible to calculate a frequency error based on them. Both methods are thus relatively unsuitable for estimation of a frequency error, since they are either relatively inaccurate or else can be carried out only with considerable effort and with great uncertainty, and thus lead to a relatively inaccurate estimate of a frequency error. Compensation for the frequency error is thus only relatively inaccurate, and is complex to carry out.